A.A.R. M-901G-82 was drafted to deal with the proliferation of heavier, 125 ton cars, and specifically the high performance draft gears being employed. In order to test and evaluate a product's suitability for this more demanding service, the railroad industry adopted a performance specification for rating a draft gear's ability to cushion the collision of one seventy-ton freight car impacting into a stationary seventy-ton car. To meet specification requirements, a collision speed of at least 5.0 MPH must be achieved while the resulting peak force (impact force) acting on each car's coupler remains below 500,000 lbs. A device which passes this threshold test is also tested statically for a resisting force of at least 8,000 lbs. Then, each test gear is subjected to substantial energy input under a drop hammer, after which each gear must repeat the same freight car collision test and again achieve at least 5.0 MPH at less then 500,000 lbs. coupler force. A further requirement is the combined draft and buff travel of the device cannot exceed six and one-half inches. To be successful, most draft gears must undergo long smooth travels during collision, taking advantage of a substantial portion of the available buff travel.
Since the adoption of the freight car impact performance specification by the industry, all existing friction-type draft gears, as well as new ones since introduced to the market, which were rated by existing drop hammer performance specifications, have been unable to meet the requirements of the freight car impact specification (AAR M-901G-82). The principal reason is the substantial increase in frictional resistance to closure exhibited by friction-type draft gears after a moderate amount of energy input. A friction-type gear that initially met the 5.0 MPH/500,000 lbs. coupler force requirement would not successfully repeat the impact test after the gear's various friction parts wore into increased contact during the drop hammer energy input portion of the specification procedure. Coupler forces would typically be greater than 500,000 lbs. at a collision speed of less than 5.0 MPH during the follow-up impact test. The essence of the problem has been that heretofore, friction-type gears did not incorporate the technology necessary to adequately control frictional resistance throughout the duration of the freight car impact specification, and therefore could not meet its requirements.
Prior to the invention described herein, only devices utilizing expensive, leakage prone hydraulic dampers in one form or another have been able to satisfy the freight car impact rating specification. Mixed type draft gears comprising both a friction mechanism and a hydraulic damper, which have met the specification, have the inherent disadvantage of added cost as well as sacrificed spring capacity, as compared to a friction-type gear having only a friction mechanism and resisting spring package. Hydraulic type gears qualified under the specification rely on internal gas pressure as a means of returning the device to datumn after an operating stroke. The drawback to such technology is the poor static load resistance provided by the gas spring, in addition to high cost. Clearly, the friction/elastomeric type draft gear of this invention is preferred for modern heavy haulage service.
A complete copy of Specification M-901G-82 may be secured from the Association of American Railroads, Mechanical Division, Manual of Standards and Recommended Practices, at 50 F Street N.W., Washington, D.C. 20001. This Specification is hereby incorporated by reference hereinto.
Up to this time, no-all friction, no all-elastomer, and no friction-elastomer draft gear has been designed which satisfactorily could meet these performance specifications; this in spite of the costs and service problems with hydraulic-type draft gears. Basic among the problems facing elastomer-type draft gears was an elastomer could not be found which provided the needed energy absorption over commercially acceptable lifetimes. Friction gears could not be made smooth enough over the typical 31/4 inch long buff travel available. Additionally, after the energy input and wearing of parts, it was found that they could not meet the second impact test involving the 70 ton rail cars moving at five miles an hour and with the needed travel while keeping the impact forces below 500,000 lbs.